... are where translation occurs. Each ribosome is a complex of proteins and special RNA called ribosomal RNA (rRNA). In both prokayotes and eukaryotes active ribosomes are composed of two subunits called the large and small subunit. Bacterial ribosomes (prokaryotic) are smaller than eukaryotic ribosomes.

In bacteria, the small subunit (called the 30S subunit) consists of 21 proteins and a length of rRNA called 16S RNA, while the large subunit (called the 50S subunit) consists of 31 proteins a length of rRNA called the 23S RNA and a length of rRNA called the 5S RNA.

The small ribosomal subunit has cleft in the upper third, and it the subunit which binds to the -AGGAGGU - sequence of the mRNA molecule. The large subunit is more complex and has two protuberances, a valley and a stalk as well as a polypeptide exit site.

When not active, ribosomes split into the two separate subunits, large and small. As protein synthesis begins, one small and one large subunit come together to form an active ribosome.